Monolithic devices have evolved quickly over their history. For example, silicon integrated circuits have grown from small scale integration (SSI) to large scale and very large scale integration (LSI and VLSI) to the current standard of ultra large scale integration (ULSI). The reason for the intense growth is the desire to implement more and more circuitry into a smaller and smaller area.
While development of integrated circuits took off, some monolithic circuit capabilities have been practically ignored. Case in point; the monolithic bipolar transistor circuit built from a discrete bipolar process. In the beginning, when solid state transistors were first manufactured, the transistors were made according to a discrete bipolar process. In this process, the wafer substrate comprised the collectors of each transistor formed on the wafer, and therefore, the transistors were not isolated from each other. If a multi-transistor circuit was to be manufactured on a single silicon wafer, the circuit designer would have to deal with the difficult design constraint of having all of the transistor collectors connected together. Thus, bipolar transistors were typically sold individually by dicing the wafer and housing each transistor in separate packages. The circuit designer would then interconnect these discrete components, along with other traditional circuit components, such as resistors, capacitors and inductors, to form multi-transistor circuits.
The demand, however, was to have multiple transistors in a single package to aid in the miniaturization of electrical devices and components. It is from this demand that today's integrated circuit industry emerged. The development of new processes allowed for the design of the metal oxide semiconductor and the CMOS chip. The CMOS technology allowed more transistors to be implemented in small chips without heat dissipation problems. As a result, the nature of circuit design changed. Inductors could not be realized on the chips, capacitors could be built, but were often avoided because their inclusion increased the cost of the chip (their size made the chip too big or their inclusion degraded manufacturing yields). Thus, circuit design evolved into a heavy utilization of transistors, thereby increasing the need to implement more transistors on a single chip.
Discrete bipolar transistor technology was not killed by the integrated circuit however. The isolation techniques required to built an integrated circuit added a substantial amount of parasitic capacitance to the transistor. This resulted in a reduction of bandwidth which was substantial enough to make integrated circuits inappropriate for microwave use. The reduction in operating speed was something that integrated circuit designers willingly accepted in exchange for the high transistor density of the integrated circuit. Microwave circuit designers, on the other hand, remained dependent upon the discrete bipolar transistor because of its ability to operate at extremely high frequencies.
Without the isolation process used in integrated circuits, multi-transistor circuit design is virtually impossible since all transistor collectors are connected together. One particular circuit configuration, however, is currently commercially available in an integrated bipolar transistor circuit. Namely, a Darlington integrated transistor pair has been achieved. The Darlington pair consists of a pair of transistors having their collectors connected together and the emitter of the first transistor is connected to the base of the second transistor. The emitter of the second transistor, the base of the first transistor and the common collector are available externally for connection.
The Darlington configuration is useful as either a very high gain amplifier, with h.sub.fe as high as 30,000, or as a high input impedance amplifier with approximately unity gain. The problem remains, however, that external components must be added to the circuit to bias the transistors. Further, the types of components that can be integrated are severely limited to bipolar transistors (with common collectors) and small resistors.